Technical Field
The present invention relates to photovoltaic devices and methods for fabrication, and more particularly to devices, structures and fabrication methods that reduce light-induced degradation and increase performance by employing a silicon buffer layer between an electrode and a p+ doped layer in a photovoltaic cell.
Description of the Related Art
When a hydrogenated amorphous silicon solar cell (a-Si:H) is soaked in solar radiation, efficiency of the cell is degraded as soaking time increases. Such phenomenon has not been observed for other types of solar cells. This is called the Staebler-Wronski effect. According to conventional theory of the Staebler-Wronski effect (SW effect), hydrogen is reconfigured increasing hydrogen passivation of silicon base materials of the solar cell under photon illumination. The solar cell experiences reduced fill factor (FF) and reduced open circuit voltage (Voc) both of which reduce the efficiency of the solar device.
A barrier height or barrier is a difference between work functions between different materials. The barrier is affected by the type of material with which a semiconductor is in contact. A band offset is the measure of misalignment between energy levels at the interface between two solids. The offset between an electrode and a semiconductor is called a “Schottky barrier”. The barriers and offsets are measures of how much a given material resists the flow of electrical charge through a medium. These quantities are also negatively affected in solar cells by light induced cell degradation. Both semiconductor-semiconductor band offset and semiconductor-electrode Schottky barrier increase the SW effect.
The barrier height between an electrode and a doped semiconductor layer may be high. To reduce the barrier height and therefore reduce Schottky barrier formation, a buffer layer may be employed. The buffer layer selected for amorphous silicon solar cells is typically Ge, which is good for reducing light degradation. However, Ge absorbs light and is prone to structural defects, which may degrade performance.